(1) Field of The Invention
The present invention relates to a servo circuit for controlling the positioning of an object under servo control (hereinafter referred to as a "servo object") to a target position by speed control and then position control.
(2) Description of the Related Art
Wide use is being made of servo positioning circuits for the positioning of magnetic heads on tracks of magnetic disk devices. In such servo positioning circuits, there is a desire for a technique enabling stable positioning even at high speeds.
Speed control is performed by supplying to a voice coil motor the difference between a target speed controlled by a difference counter decremented by a track crossing pulse and a speed signal prepared from a differential signal obtained from a position signal and a current detection signal.
In the preparation of the control speed, with a high speed seek (movement), the number of position signals per unit time is high, and along with this there are many differential signals generated, so the gap between the speed signal prepared by the circuit and the actual speed is not that large. However, when the speed becomes low, the number of differential signals per unit time falls and the gap with the actual speed increases.
To cover for this, the current detection signal is being added to, but no great improvement can be expected from this. That is, the maximum speed has been rising more and more in recent years so as to enable high speed seek operations, but the amount of adding of the current detection signal cannot be increased along with the speed. The reason is that during a high speed seek operation, the current detection signal is not required, so it is not possible to add a large enough amount as to have an impact on the high speed seek operation.
This difference with the actual speed during low speed operation is particularly a problem in a deceleration process. A problem has arisen in which the following characteristic of the servo object falls when deceleration is completed. Therefore, a possibility has arisen of overshooting or undershooting occurring due to relaxation of the magnetic flux of the voice coil motor, effects of weight of the voice coil motor in the seek direction, air flow due to rotation of the magnetic disk, or other outside disturbances.